Subsea remotely operated chain tensioning and slacking system

ABSTRACT

A mooring line tension adjusting system comprises a connecting device, connecting and locking two adjacent chain sections of a mooring line, and an operating device, moving one of the chain sections inside the connecting device to change the tension of the mooring line. The system has the inventive feature that the operating device is remotely attachable to the connecting device before a tensioning operation and detachable from the connecting device after the tensioning operation. The invention discloses also a method how to use the tensioning system.

FIELD OF INVENTION

The present invention relates to offshore mooring systems. More specifically it relates to a subsea, remotely operated tension adjusting system for mooring lines where the lines comprise chain sections.

INTRODUCTION AND BACKGROUND

Definitions and Abbreviations

The abbreviations used in this document are:

DFF Design Fatigue Factor

EOL End of Life

ESD Emergency Shut Down

FPSO Floating Production Storage and Offloading

FSO Floating Storage and Offloading

FSU Floating Storage Unit

HPU High Pressure Unit

HS Hot Spot

MBL Minimum Breaking Load

MLBE Mooring Line Buoyancy Element

MSL Mean Seawater Line

MTF Mechanical Transfer Function

ROV Remotely Operated Vehicle

STL Submerged Turret Loading

STP Submerged Turret Production

SWL Safe Working Load

TBD To Be Determined

WROV Work ROV

The definitions used in this document shall be understood as follows:

Buoy: Complete STL/STP Buoy comprising:

Buoyancy Cone, Bearings, Turret, ESD valves, Buoy part of Hydraulic and Signal Connectors, Riser hang-offs and connections, Mooring connections and Pick-up Assembly.

Connecting device: Chain connecting device that is permanently/fixed located in the mooring line; also simply ‘Connector’

Operating device: Tool to operate the connecting device during tension adjusting operation; attachable to and detachable from the connecting device.

Tensioning system: System comprising connecting device and operating device; also called ‘tensioner’.

Mooring System: Complete mooring system comprising: Anchors, Chain, Wire, Polyster Rope, Mooring Line Buoyancy Elements and Connections.

Riser System: Flexible riser and umbilical system from seabed to vessel.

Riser: Flexible riser for transfer of liquids and gases.

Subsea System: Field related system comprising: Mooring, Buoy and Riser/umbilical System.

Umbilical: Flexible umbilical for power/hydraulic and signal lines.

Mooring systems in deep and ultradeep waters often require use of polyester ropes, because of weight issues and vessel offset limitations. One drawback with polyester ropes is however that it creeps over time when subject to continuous loading. It also creeps when it experiences loads higher than it has seen earlier. Part of this creep can be mitigated by stretching the rope to a high tension during the offshore installation campaign, but for practical reasons (installation vessel capabilities and safety) there is an upper limit on how much tension that can be applied. Hence, re-tensioning of polyester systems will most likely be required regularly over the design life of the field.

Buoys do often not have any re-tensioning possibility, because that feature would grow the size and the complexity of the Buoy. Re-tensioning has therefore been done as a combination of tensioning during installation (to typically 20-30% of the MBL for the polyester rope), and by opening the line, cutting chain and closing the line again, if later re-tensioning is required. The latter operation may however be expensive, as it typically requires large vessel(s) with significant winch/crane capacity, ROV, weather limitations, long planning due to limited number of vessels that can do the operation, etc.

There exists thus a need for a new tensioning system in order to simplify and reduce the cost/risk of the present tension adjusting methodology.

SUMMARY

The present invention is a mooring line tension adjusting system comprises a connecting device, connecting and locking two adjacent chain sections of a mooring line, and an operating device, moving one of the chain sections inside the connecting device to change the tension of the mooring line. The system has the inventive feature that the operating device is remotely attachable to the connecting device before a tensioning operation and detachable from the connecting device after the tensioning operation.

One of the parts comprised in the system is a connecting device, comprising a first connection arrangement for a first chain and a second connection arrangement for a second chain wherein the first connection arrangement provides a permanently fixed connection of said first chain and the second connection arrangement provides a connection which can be modified by an operating device. The connecting device comprises a first docking element—for instance a pin—enabling the operating device to firmly attach to the connecting device prior to performing a tension adjusting operation.

The second part of the system is an operating device with a second docking element being compatible with the first docking element, enabling the operating device to firmly attach to the connecting device to perform a tension adjustment operation.

A further aspect of the invention is a method for tensioning the mooring line with the tension adjusting system disclosed above. The method comprises

a. positioning a surface vessel above the mooring line, slightly to the side of a connecting device, where two sections of the mooring line are connected by the connecting device;

b. attaching one end of a first chain or similar elongated element to one of two attachment points provided on a guide at an lower end of the operating device and attaching a first weight bar to the second end of the first chain;

c. attaching a second chain or similar element, longer than the first chain to a second attachment point on the guide at the lower end of the operating device and attaching a second weight bar to the second end of the second chain;

d. attaching a lifting/handling equipment to a third attachment point at an upper end of the operating device;

e. connecting an umbilical to a power system of the operating device, and to an power supply system on board of the surface vessel;

f. overboarding the operating device with the umbilical and accessories and lower it slightly above the same depth as the connecting device mounted into the mooring line;

g. moving the vessel such that the second chain hits the connecting device, and by moving the vessel slightly passed but without the second chain jumping over the operating device should rotate such that it orients correctly relative to the connecting device;

h. lowering the operating device such that the connecting device is between the first and the second chain;

i. continuing to lower the operating device until it sits on top of the connecting device;

j. after the lower end of the operating device has docked onto the connecting device sliding it down along the connecting device until it stops against a first docking element, a second docking element on the operating device connecting around the first docking element;

k. operating the operating device via the umbilical for pushing the mooring line in a tensioning direction; repeating this step until a requested tension is achieved;

1. lifting the operating device off the connecting device, unhooking from the first docking element.

BRIEF DESCRIPTION OF THE DRAWINGS

Below the invention will be described in detail with reference to the attached figures. Contents of the figures as follows:

FIG. 0.1 Typical mooring line composition including both rope and chain parts. The inventive tensioning system may replace OS, UCS2 and SP

FIG. 0.2 Location of the tensioning system/connector in the mooring system

FIG. 0.3 Assembly of the connecting device and the operating device of the tensioning system

FIG. 0.4 Fixed tensioner part with illustration of force direction from chain onto locking elements

FIG. 0.5 Connecting device of tensioning system

FIG. 0.6 General arrangement of the connecting device of the tensioning system

FIG. 0.7 Operating device of the tensioning system

FIG. 0.8 General arrangement of operating device of tensioning system

FIG. 0.9 Docking of operating device onto connecting device

FIG. 0.10 Illustration of fixed and operating device including tensioned chain and free end

FIG. 0.11 Illustration of docking of operating device onto connecting device

FIG. 0.12 Tensioning operation

FIG. 0.13 Slackening operation

FIG. 4.1 Mid Line Tensioning system

FIG. 4.2 Mid Line Tensioner Connecting device

FIG. 4.3 Mid Line Removable Tool

FIG. 4.4 Locking Element in Connecting device

FIG. 4.5 Pushing Element in Operating device

DETAILED DESCRIPTION

Main goal of this inventive concept is to manage regular tensioning adjustments of mooring lines—both tensioning and relaxing, in order to stay within the design envelope of the mooring system. The tensioning system shall not require a huge offshore campaign, and the operation shall be done with a relatively small vessel in combination with an ROV, without opening the mooring line. The re-tensioning operation is planned conducted with a vessel with minimum crane or A-frame capacity, but equipped with an ROV/WROV that can observe the operation as well as operate the power (normally hydraulics) of the tensioning system. Alternatively, the power can be operated via an umbilical between the tensioner and the vessel.

The inventive tensioning system comprises two main parts; confer FIG. 2.1 for a general illustration and FIGS. 4.1, 4.2, 4.3, 4.4, 4.5 for details:

-   -   Connecting device; this component becomes a permanent/fixed part         in the mooring line     -   Operating device; this is a tool, used (only temporary) for the         tensioning/slacking process, also called ‘removable part’

The operating device of the tensioner can be mounted/docked on top of the connecting device through a hook or similar arrangement at the lower end. During the tensioning process the pushing element of the operating device pushes the upper chain towards the lower end of the connector. The movement of the chain unlocks the locking element of the connecting device, which remains unlocked until the chain has moved far enough for the locking element to drop down by gravity and thereby lock the chain again. The pushing element can then be retreated such that it can take a new grip and repeat the sequence until the mooring line has been shortened to the desired length. Each cycle will typically move two chain links.

A corresponding operation can also be used to lengthen the mooring line thus reducing the tension, but this requires that the locking elements of the connecting device are lifted by the ROV when the pushing element has off-loaded the contact between the chain and the locking elements. Otherwise it will lock the chain from being moved backwards.

The tensioner will most likely be located above or below the Mooring Line Buoyancy Element (MLBE), as shown in FIG. 1.1 and FIG. 1.2.

The reason for dividing the tensioner into a fixed and a removable part is to minimize the permanent weight in the mooring system and thereby minimize the required buoyancy of the MLBE and the STL/STP Buoy. It will also reduce the overall cost since the same removable part can be used for all mooring lines, and maintenance of the hydraulic parts and the mechanical components will be easier.

A fixed weight for instance a hinged rod is attached to the connecting device in order to lower the overall centre of gravity and thus ensure that the connecting device is always upright. This weight may not be required.

The locking elements for the chain can be kept in place by plates on each side as well as a one-sided bolt with threads on one side and threadless and headless on the other side. This bolt is entered through the outer tensioner wall into the locking element and locked inside the element preferably with a ZipNut; see FIG. 2.6.

When the bolt is fully fixed it is flush with the outer tensioner wall and free to rotate inside the hole in the wall. There is clearance between the wall opening and the bolt such that the load from the chain into the locking element and further into the support structure does not stress the bolt. Main reason for using the ZipNut technology is to be able to relatively easily replace the locking element with a WROV.

Hydraulically operated cylinders f. inst can be mounted on the operating device, preferably one on each side of the tool, in order to provide the required force to push the chain. Total available force from two hydraulic cylinders could be 150 tonnes, which means that each unit would have to provide minimum 75 tonnes. The cylinder units will be connected together such that they provide the same push at the same time. Hydraulic pressure could be provided via a WROV or directly via an umbilical from the surface.

The tension adjusting system can be protected against corrosion in order to avoid any degradation of the functionality due to the marine environment. However, since the operating device typical is only used temporarily, this protection needs only be considered to be applied to the connecting device.

Operation Procedure

The following provides a possible high level description of the tensioning operation. The main steps in the tensioning operation would be:

1) Position a surface vessel above the mooring line, slightly to the side of the connecting device.

2) Attach a chain or similar to one of the padeyes on the V-shaped guide at the end of the operating device. Attach a weight bar to the end of the chain.

3) Attach a longer chain or similar to the other padeye on the V-shaped guide at the end of the operating device. Attach a weight bar to the end of the chain.

4) Attach a lifting/handling wire to the padeye at the upper end of the operating device.

5) Connect the (typically hydraulic) umbilical to the power system of the operating device, and to power providing unit on board the surface vessel.

6) Overboard the operating device with the umbilical and the chain accessories and weight bars and lower it to almost the same depth as the connecting device mounted on the mooring line. Pay out the umbilical accordingly. The operating device should now be positioned slightly to the side (the side depends on which side of the operating device the longer chain is connected) of the connecting device, with the lower end of the weight bar connected to the short end above the connecting device and the other bar below the fixed bar.

7) Move the vessel such that the longer chain hits the connecting device. By moving the vessel slightly passed but without the long chain jumping over the operating device should rotate such that it orients correctly relative the connecting device.

8) Lower the operating device such that the connecting device is between the two chain segments; confer FIG. 3.1 and FIG. 3.3. Continue to lower it until the operating device sits on top of the connecting device. After the lower end of the operating device has docked onto the connecting device it will slide down the connecting device until it stops against the pin, which the hook shall connect to. Continue to pay out until the operating device has fully docked, and the hook is fully engaged around the pin. The hook will connect to the pin at a relative angle of 30 to 45 degrees between the fixed and operating device; 0 degrees is when they are fully latched together.

9) With the supervision of an ROV, operate the hydraulic units via the umbilical to the surface vessel for the pushing the chain. Number of cycles of 2-link pushes depends on how much total length adjustment that is required, confer the pushing sequence shown in FIG. 3.4

10) When the tensioning has been completed the operating device is lifted off the connecting device, and either lifted onboard the surface vessel or moved to the next mooring line.

For slackening the system the same procedure can be used, but the cylinder pistons are operated in the opposite direction, confer FIG. 3.5. Another difference is that when the pushing segment release the stresses on the locking elements by pushing at the far end of the 2-link grip the locking elements on the connecting device must be lifted/opened by the ROV; otherwise the locking elements will prevent the chain from being moved backwards. The ROV will let the locking elements engage with the mooring chain as soon as the first link has passed underneath the element. 

1. A mooring line tension adjusting system, comprising a. a connecting device, connecting and locking two adjacent chain sections of a mooring line, and b. an operating device, moving one of the chain sections inside the connecting device to increase or relax a tension on the mooring line, wherein the operating device is remotely attachable to the connecting device using docking elements provided on both the connecting device and the operating device before a tensioning operation and the operating device is detachable from the connecting device after a tension adjustment operation.
 2. A connecting device for use in the mooring line tension adjusting system according to claim 1, comprising a first connection arrangement for a first chain and a second connection arrangement for a second chain wherein the first connection arrangement provides a permanently fixed connection of said first chain and the second connection arrangement provides a connection which can be modified by an operating device, wherein the connecting device comprising a first docking element enables the operating device to firmly attach to the connecting device prior to performing a tension adjusting operation.
 3. An operating device for use in the mooring line tension adjusting system according to claim 1, comprising a second docking element being compatible with a first docking element on a connecting device comprising a first connection arrangement for a first chain and a second connection arrangement for a second chain wherein the first connection arrangement provides a permanently fixed connection of said first chain and the second connection arrangement provides a connection which can be modified by an operating device, wherein the connecting device comprising a first docking element enables the operating device to firmly attach to the connecting device prior to performing a tension adjusting operation, enabling the operating device to firmly attach to the connecting device to perform a tension adjustment operation.
 4. A method for tensioning a mooring line with the tension adjusting system according to claim 1, said system comprising a connecting device connecting two chains and an operating device, the method comprising the steps of a. positioning a surface vessel above the mooring line, slightly to the side of a connecting device, where two sections of the mooring line are connected by the connecting device; b. attaching one end of a first chain to one of two attachment points provided on a guide at an lower end of the operating device and attaching a first weight bar to the second end of the first chain; c. attaching a second chain, longer than the first chain to a second attachment point on the guide at the lower end of the operating device and attaching a second weight bar to the second end of the second chain; d. attaching a lifting/handling equipment to a third attachment point at an upper end of the operating device; e. connecting an umbilical to a power system of the operating device, and to an power supply system on board of the surface vessel; f. overboard the operating device with the umbilical and accessories and lower it to slightly above the same depth as the connecting device mounted into the mooring line; g. moving the vessel such that the second chain hits the connecting device, and by moving the vessel slightly passed but without the second chain jumping over the operating device should rotate such that it orients correctly relative to the connecting device; h. lowering the operating device such that the connecting device is between the first and the second chain; i. continuing to lower the operating device until it sits on top of the connecting device; j. after the lower end of the operating device has docked onto the connecting device, sliding it down along the connecting device until it stops against a first docking element, a second docking element on the operating device connecting to the first docking element; k. operating the operating device via the umbilical for pushing the mooring line in a tensioning direction; repeating this step until a requested tension is achieved; and l. lifting the operating device off the connecting device, unhooking from the first docking element. 